Although the weather may sometimes seem unkind to us in Ireland, we should be grateful that there are some disasters with which we do not have to cope. In countries where there are large amounts of winter snow, for example, avalanches are a major source of danger every year. They are also a serious hazard to skiers, since the conditions most tempting for the expert are often to be found in precisely those places where avalanches are most likely to occur.
Statistically speaking, however, an avalanche is a pretty harmless thing. Compared to other natural hazards, they cause relatively few fatalities and do little harm to property. But these small losses do not stem from any scarcity of avalanches around the world; the reason is that the vast majority occurs in uninhabited areas, and for every one that hits the news, a thousand others cascade down their own secluded mountains, incognito.
Most avalanches happen on slopes with a gradient of between 30 and 45 degrees. As a general rule the "north wind's masonry" first arranges the "frolic architecture of the snow" in such a way as to bring about a structure which is unstable; all it needs then is some mechanical or meteorological trigger to send thousands of tonnes of snow cascading down the mountainside at staggering speeds. Dry snow can move at up to 200 m.p.h.; wet snow travels more slowly, but causes greater damage because of its greater weight.
If conditions are right, an avalanche can be triggered by a loud noise. Indeed, in some countries explosions are detonated in order to start avalanches in controlled conditions - to get them over with, as it were. But even the passage of a single skier across the surface of the snow may sometimes be enough to provide the necessary stimulus.
Meteorological factors are also important. An increase in temperature often causes a weakening of the bonds between the individual ice granules; if the temperature rise continues, it may result in a thin film of water around each granule, which acts as a lubricator and facilitates movement. This effect is greatest with an approaching thaw, so springtime is often a dangerous season. And for the same reason, avalanches often occur most readily on slopes which catch the sun.
Rain or fresh snow can be a trigger. Rain percolates into the layers of snow beneath the surface, and weakens the bonds between the successive layers; it also increases the shearing force, by adding to the weight of the topmost snow. And fresh snow, too, adds extra weight to the surface, and may be sufficient to result in instability.